1. Field of the Invention
The present invention relates to a battery charge level detecting device for detecting the charge level of a secondary battery.
2. Description of the Related Art
An electric vehicle (including a hybrid electric vehicle) obtaining the total or a part of the driving force of the vehicle by an electric motor, has a secondary battery (hereafter, referred to simply as a battery) mounted on the vehicle, and by the electric power accumulated in this battery, said electric motor is driven. Regenerative braking is a characteristic function in such an electric vehicle. In regenerative braking, during vehicle braking, the kinetic energy of the vehicle is transformed into the electric energy by making said electric motor function as an electricity generator. The obtained electric energy is accumulated in the battery and is reused for acceleration or other vehicle needs. Accordingly, with regenerative braking, it is possible to reuse energy is normally radiated into the atmosphere as thermal energy in a conventional automobile which runs by an internal combustion engine alone, and the efficiency of the energy can considerably be improved.
Here, in order to effectively accumulate the electric power generated during regenerative braking in a battery, it is necessary for the battery to have a corresponding margin of capacity. Furthermore, in a type of hybrid electric vehicle in which the generator is driven by the heat engine mounted on the vehicle and generates the electric power and this electric power can be accumulated in a battery, the electric power accumulated in the battery, that is, the charge level can freely be controlled. Consequently, in such a hybrid electric vehicle, it is desirable that the charge level of a battery be controlled such that the charge level is approximately in the middle state (50.about.60%) between the state of full charge (100%) and the state of no charge (0%), so that the regenerative power may be received, and so that the electric power may be supplied to the electric motor immediately if a request is made. Accordingly, it is necessary to accurately detect the charge level of a battery.
Methods to detect the charge level of a battery on the basis of the terminal voltage of the battery are well known. However, during the charge or discharge, the terminal voltage changes with the value of the current, even when the charge level is the same. Therefore, the charge level of a battery cannot accurately be detected by the terminal voltage alone. Accordingly, if raising the detection accuracy of the charge level is desired, a method to detect the charge level on the basis of both the current flowing in the battery and the terminal voltage is used. Such a method to detect the charge level is disclosed in Japanese Patent Laid-Open Publication No. Hei9-72984.
Furthermore, depending on the type of a battery, there is a known battery in which the charge level does not affect the external characteristics of the battery in some area of the charge level, and in this case, a method to estimate the charge level by integrating the currents of the charge and discharge, is adopted.